A hydrogenated norbornene ring-opening polymer has excellent transparency and a low birefringence, and application thereof as a resin material for an optical lens or an optical sheet has been proposed (Patent Documents 1 and 2). Since the hydrogenated norbornene ring-opening polymer exhibits excellent fluidity in a molten state and has excellent eluting properties and chemical resistance, application thereof as a resin material for other applications such as a packing film or a medical container has also been proposed (Patent Documents 3 and 4).
However, since most of the hydrogenated norbornene ring-opening polymers disclosed in these documents are amorphous, their water vapor barrier properties, oil resistance, and the like are insufficient depending on the application. Therefore, a further improvement in properties of hydrogenated norbornene ring-opening polymers has been desired.
Patent Document 5 discloses a method that obtains a hydrogenated amorphous norbornene ring-opening polymer by ring-opening polymerization of a norbornene monomer having an alkenyl group with a carbon-carbon double bond at the chain end and a norbornene monomer which does not have an alkenyl group with a carbon-carbon double bond at the chain end, and hydrogenating the resulting polymer. Patent Document 5 describes that the resulting hydrogenated amorphous norbornene ring-opening polymer has excellent moldability and is suitable for forming an optical molded article by injection molding.
However, the amorphous polymer disclosed in Patent Document 5 has poor moisture resistance.
As a hydrogenated norbornene ring-opening polymer having crystallinity (i.e., having a melting point), a hydrogenated crystalline norbornene ring-opening polymer containing a repeating unit of a norbornene monomers having three or more rings has been known (Patent Documents 6 to 8). A resin film or sheet obtained using the hydrogenated norbornene ring-opening polymer disclosed in these documents has excellent transparency, heat resistance, chemical resistance, and mechanical strength.
However, a film obtained by molding the above hydrogenated crystalline norbornene ring-opening polymer does not fully satisfy the requirements for moisture permeability. Moreover, the above hydrogenated crystalline norbornene ring-opening polymer has poor solubility in a solvent, and may precipitate from a reaction solution after hydrogenating the ring-opening polymer. This may make it difficult to sufficiently purify the polymer by removing residual catalysts and the like.
Non-patent Documents 1 and 2 disclose a hydrogenated norbornene ring-opening copolymer possessing crystallinity. However, these documents do not specifically describe the properties of the polymer. Among the specifically disclosed polymers, the polymers having a high molecular weight and a narrow molecular weight distribution exhibit difficulty in producing a film with a smooth surface, since these polymers tend to undergo melt-fracture due to high shearing viscosity at a high shear rate during film formation. On the other hand, a film formed of a polymer having a low molecular weight has small tensile elongation at break (i.e., insufficient mechanical characteristics). Moreover, since the hydrogenation degree of the hydrogenated ring-opening polymer disclosed in these documents is not necessarily sufficient, a molded article obtained by molding the polymer may be easily discolored.    Patent Document 1: JP-A-60-26024    Patent Document 2: JP-A-9-263627    Patent Document 3: JP-A-2000-313090 (WO 2000/066357)    Patent Document 4: JP-A-2003-183361 (EP 559146)    Patent Document 5: JP-A-2007-262170    Patent Document 6: JP-A-2002-020464    Patent Document 7: JP-A-2002-194067    Patent Document 8: JP-A-2006-052333    Non-patent Document 1: Polymer International, Vol. 34, pp. 49-57 (1994)    Non-patent Document 2: Macromolecules, Vol. 37, pp. 7278-7284 (2000)